Goldfish Herpesvirus (CyHV-2): Herpesviral Haematopoietic Necrosis

CyHV-2 causes herpesviral haematopoietic necrosis, a highly lethal disease of goldfish. Healthy-looking carriers are common in the trade, and temperature swings can trigger outbreaks.

Herpesviral haematopoietic necrosis (HVHN) is a viral disease of goldfish (Carassius auratus) caused by cyprinid herpesvirus 2 (CyHV-2). It destroys the blood-forming tissue of the kidney and spleen, can kill most of a group within days to weeks at moderate water temperatures, and has no cure; surviving fish remain carriers.

Cause

CyHV-2 is a herpesvirus of the family Alloherpesviridae (species name Cyvirus cyprinidallo2). The disease was first described in Japan by Jung and Miyazaki in 1995 after epizootics in 1992 and 1993. It is now reported in Asia, Europe, North America and Australia; in the UK it was isolated from goldfish at an ornamental retail site in southern England in 2004. Goldfish of all ages are susceptible, and the virus also causes outbreaks in Prussian carp (Carassius gibelio) and crucian carp. It is a different virus from carp pox (CyHV-1) and koi herpesvirus (CyHV-3), which are covered in separate articles.

Clinical signs

• Lethargy, loss of appetite and inactivity.
• Pale gills, sometimes with pale patches on the gills and skin.
• Fluid accumulation in the body cavity.
• In some outbreaks, haemorrhages and skin ulcers.
• Internally: an enlarged spleen and kidney, often pale with white nodules or necrotic areas; necrosis may also affect the pancreas, intestine and gill lamellae.

These signs are not specific. Fish can die with few external changes, and other infections can look similar, so the cause must be confirmed by a laboratory.

Temperature range

Water temperature strongly controls the disease. The MSD Veterinary Manual gives viral replication between 10 and 22 °C, a UK study describes typical infection of goldfish at 20–25 °C, and the Ontario Animal Health Network cites 15–25 °C. In experimental infections of Ryukin goldfish, cumulative mortality was 10% at 15 °C, 90% at 20 °C and 25 °C, and 60% at 30 °C, so 20–25 °C was considered highly permissive. Fish infected at 13–15 °C mostly survived but became carriers that could infect naive fish.

Temperature changes appear to trigger outbreaks. On commercial farms, a sharp drop from 22 to 10 °C raised virus levels in healthy carriers within 24 hours, while heat, ammonia and high pH did not; outbreaks occurred when carriers were chilled and then held at a permissive temperature. In the UK case, signs started after water warmed from 14–15 to 19–21 °C. In crucian carp, nitrite stress lowered survival during experimental infection.

Mortality in goldfish

Outbreaks in cultured and wild goldfish are described as highly lethal. A fish kill in a New Jersey lake in June 2013 killed an estimated 3,000–5,000 adult goldfish within about five days. The Ontario Animal Health Network notes high mortality in goldfish younger than one year. An isolate from crucian carp caused rapid, fatal disease in goldfish at 28 °C under experimental injection.

Carriers and spread

Apparently healthy goldfish often carry the virus. In a survey of commercial goldfish farms, healthy fish from 14 of 18 farms tested positive, and the virus was found in fry produced from disinfected eggs. After infection, viral DNA persisted in the spleen, kidney and gills of survivors for at least 81 days, and survivors could infect other fish. In the Netherlands, CyHV-2 DNA was found in 4 of 8 batches of goldfish imported by air from Asia and the Middle East. In Australia, the virus became established in retail, farmed and wild goldfish despite health certification and three weeks of import quarantine, and the requirement for CyHV-2-free certification was later withdrawn.

Diagnosis

A veterinarian or fish-health laboratory confirms HVHN by PCR on kidney, spleen or gill tissue from sick or freshly dead fish, supported by histopathology showing necrosis of the kidney and spleen with characteristic nuclear changes. Electron microscopy can show herpesvirus particles, even in formalin-fixed tissue. Because carriers can test positive without disease, results are interpreted together with signs and history.

Treatment and its limits

There is no treatment for the virus. The MSD Veterinary Manual reports that raising the water temperature above the permissive range resolves clinical signs, but fish remain lifelong carriers and can relapse under stress. Experimental infections still caused high mortality at 28–30 °C, so temperature manipulation is not a reliable cure and should only be done on the advice of an aquatic veterinarian. Vaccines have been developed and tested in research settings.

Biosecurity for goldfish keepers

• Treat every new goldfish as a possible carrier: quarantine new arrivals in a separate system with separate nets and buckets, and do not add them to an established group without veterinary advice if any deaths occur.
• Remember that quarantine and health certificates do not guarantee freedom from CyHV-2, because carriers look healthy.
• Avoid sudden temperature drops and swings, which have been linked to outbreaks in carriers.
• Keep water quality stable; nitrite exposure worsened survival in experimentally infected crucian carp.
• Disinfect equipment between systems and never release goldfish or tank water into the wild, where the virus has become established in wild populations.
• If fish die, have fresh dead or moribund fish examined by a fish-health laboratory rather than guessing.